BANGKOK, Jan 25 (Reuters) - Animals that have previously been vaccinated against seasonal flu appear to respond far quicker to experimental H5N1 bird flu vaccines, a study has found.

Many doctors believe that seasonal flu vaccines offer little or no protection against the H5N1 virus, which experts say may unleash a pandemic that could kill millions of people.

But a study by biotechnology firm MedImmune Inc <MEDI.O>, which produces influenza vaccine, found that ferrets that had been vaccinated against seasonal flu appeared to be more responsive when they were later administered the H5N1 vaccine.

"If you have previously received normal seasonal flu vaccine, you may have better response to the H5N1 vaccine," MedImmune's scientist Hong Jin told a bird flu conference in Bangkok.

Researchers at the National Institute for Infectious Diseases in Rome published a study in the journal Emerging Infectious Diseases in December showing that ordinary seasonal flu vaccines may provide a small amount of protection against bird flu.

Their study was among the first to support the idea that getting an annual flu shot may help people's bodies fight off the H5N1 virus.

In the laboratory, they added H5N1 virus to the blood and found that in some of the volunteers immune system proteins called antibodies acted against the bird flu virus.

In the MedImmune study, a group of ferrets was given seasonal flu vaccine, while a control group of ferrets were given nothing.

Forty days later, both groups were given H5N1 vaccines designed using seed virus taken from outbreaks of the disease in Hong Kong in 2003 and Vietnam in 2004.

They were then monitored for the production of H5N1 antibodies.

"We found much more response in ferrets that received (seasonal flu) vaccine before, whereas in control ferrets, you don't see the response," Hong told Reuters.

Huge volumes of antibody producing cells were seen in the ferrets that had both vaccines on day 45, but there was no antibody response in the control group, MedImmune said.

"Maybe the (seasonal flu) vaccine can induce immune response that can speed up response to H5 vaccine," Hong explained.

Both types of vaccines were administered using nasal spray. MedImmune first introduced its nasal spray influenza vaccine in 2003. (Reporting by Tan Ee Lyn; Editing by Caroline Drees)

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